Bus arbiter including programmable request latency counters for varying arbitration priority

ABSTRACT

A computer system is provided for controlling the ownership of a bus to which a variety of both real time and non-real time resources are coupled. The bus arbiter includes a request detection unit for detecting bus request signals of a plurality of bus masters, and a grant generator for generating corresponding grant signals to indicate a grant of ownership of the bus. A set of counters referred to as &#34;request latency&#34; counters is further provided wherein a separate counter unit corresponds to each bus master. Each counter is configured to generate a signal indicative of a lapse of time since a time when the peripheral requested ownership of the bus. An arbitration control unit is coupled to the request latency counters, the request detection unit and the grant generator for processing incoming bus request signals. The arbitration control unit is configured to dynamically vary the level of arbitration priority given to each peripheral device based upon the latency signal corresponding to the device. Accordingly, as the time from when a peripheral device requests the bus increases, the level of arbitration priority given to that peripheral also increases. A set of programmable registers are provided to allow software programming of the initial count value associated with each request latency counter. The request latency counter for a particular device may further be held or inhibited from counting to provide a constant level of priority for that particular peripheral device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to bus arbitration within computer systems and, more particularly, to a computer system having an improved bus arbiter for arbitrating bus accesses of a CPU, real time DSP hardware, and other system resources.

2. Description of the Relevant Art

Computer architectures generally include a plurality of devices interconnected by one or more buses. For example, conventional computer systems typically include a CPU coupled through bridge logic to a main memory. The bridge logic also typically couples to a high bandwidth local expansion bus, such as the Peripheral Component Interconnect (PCI) bus or the VESA (Video Electronics Standards Association) VL bus. Modern local bus standards such as the PCI bus and the VL bus are not constrained by a requirement to be backwards compatible with prior expansion bus adapters and thus provide much higher throughput than older expansion buses. Examples of devices which can be coupled to local expansion buses include SCSI adapters, network interface cards, video accelerators, audio cards, telephony cards, etc. An older-style expansion bus may also be coupled to the local expansion bus to provide compatibility with earlier-version expansion bus adapters. Examples of such expansion buses include the industry standard architecture (ISA) bus, also referred to as the AT bus, the extended industry standard architecture (EISA) bus, and the microchannel architecture (MCA) bus. Various devices may be coupled to this second expansion bus, including a fax/modem, sound card, etc.

A CPU local bus arbiter as well as a PCI bus arbiter are typically included as part of the bridge logic in many computer systems. The CPU local bus arbiter determines and prioritizes ownership of the CPU local bus, while the PCI bus arbiter determines and prioritizes ownership of the PCI bus. Mastership of either bus is typically based on a fixed arbitration fairness scheme, such as a round-robin algorithm. In some situations, a master must acquire ownership of both the PCI bus and the CPU local bus before it can proceed with a particular transfer cycle.

Computer systems were originally developed for business applications including word processing and spreadsheets, among others. Recently, computer systems have evolved toward more real time applications, including multimedia applications such as video and audio, video capture and playback, telephony, and speech recognition. Computer systems originally designed for business applications, however, are not well suited for the real time requirements of modern multimedia applications for a variety of reasons. For example, current operating systems for personal computers are not real time operating systems. In addition, the bus architecture of modern personal computer systems still presumes that the majority of applications executing on the computer system are non-real time, business applications such as word processing and/or spreadsheets which are executed solely by the main CPU.

A significant problem associated with modern computer systems is that the bus arbiter which arbitrates between real time resources, non-real time resources and the CPU is typically designed to provide the CPU with maximum availability to the system memory, or is otherwise not cognizant of the real time accesses of other bus mastering devices. Real time devices can accordingly be "starved" for memory access, particularly when a relatively large number of real time devices are included within the system. This can correspondingly result in degraded performance, unsynchronized audio and video, and the dropping of frames during video or animation sequences. Therefore, a new bus arbiter system and method are desirable which provide greater access to bus subsystems by real time devices to thus better facilitate real time applications.

SUMMARY OF THE INVENTION

The problems outlined above are in large part solved by a bus arbiter including programmable request latency counters to dynamically vary arbitration priority in accordance with the present invention. In one embodiment, a computer system is provided that includes a bus arbiter for controlling the ownership of a bus to which a variety of both real time and non-real time resources are coupled. The bus arbiter includes a request detection unit for detecting bus request signals of a plurality of bus masters, and a grant generator for generating corresponding grant signals to indicate a grant of ownership of the bus. A set of counters referred to as "request latency" counters is further provided wherein a separate counter unit corresponds to each bus master. Each counter is configured to generate a latency signal or value indicative of a lapse of time since a time when the peripheral requested ownership of the bus. An arbitration control unit is coupled to the request latency counters, the request detection unit and the grant generator for processing incoming bus request signals. The arbitration control unit is configured to dynamically vary the level of arbitration priority given to each peripheral device based upon the latency signal corresponding to the device. Accordingly, as the time from when a peripheral device requests the bus increases, the level of arbitration priority given to that peripheral also increases. A set of programmable registers are provided to allow software programming of the initial count value associated with each request latency. The request latency counter for a particular device may further be held or inhibited from counting to provide a constant level of priority for that particular peripheral device. Since the level of arbitration priority given to a peripheral device may be based upon the length of time the peripheral has been waiting to gain ownership of the bus, improved overall system performance may be obtained, particularly for real time processing environments.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:

FIG. 1 is a block diagram of a computer system including a variety of real time resources and a bus arbiter in accordance with the present invention.

FIG. 2 is a block diagram which depicts internal portions of the bus arbiter of FIG. 1.

FIGS. 3A and 3B are diagrams illustrating portions of the bus arbiter of FIG. 2 which illustrate exemplary values generated by the counters of various peripheral devices.

FIG. 4 is a block diagram of an embodiment of a bus arbiter wherein a set of registers for each bus master is provided to specify latency, bandwidth, and other operational parameters.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a block diagram of a computer system incorporating a plurality of real time bus devices and bus arbitration logic in accordance with the present invention is shown. As illustrated in the figure, the computer system includes a central processing unit (CPU) 102 which is coupled through a CPU local bus 104 to a host/PCI/cache bridge 106. The bridge 106 includes memory control logic and is coupled through a memory bus 108 to a main memory 110. A cache memory subsystem (not shown) may further be coupled to bus bridge 106.

Bus bridge 106 also interfaces to a peripheral component interconnect (PCI) bus 120. Further details regarding PCI bus 120 may be found within the publication "PCI Local Bus Specification"; Revision 2.0; Apr. 30, 1993; PCI Special Interest Group; Hillsboro, Oreg. This publication is incorporated herein by reference in its entirety. It is noted that other local buses could be alternatively employed, such as the VESA (Video Electronics Standards Association) VL bus.

CPU 102 is illustrative of, for example, an x86 microprocessor such as an 80486 microprocessor or a Pentium-compatible microprocessor. It is understood, however, that a system according to the present invention may employ other types of microprocessors. It is further understood that the present invention may be employed within a multiprocessing environment.

Various types of devices may be connected to the PCI bus 120. For the embodiment illustrated in FIG. 1, a video adapter 170 for controlling video functions is coupled to PCI bus 120. Other real time DSP devices are also preferably coupled to the PCI bus, including an audio adapter 172, a telephony adapter 174, and a video capture board 176, among others. A SCSI (small computer systems interface) disk controller 122 and a network interface card 140 are additionally shown coupled to the PCI bus 120. SCSI controller 122 is configured to provide an interface to SCSI devices such as a CD-ROM device, a tape drive device, and/or a composite disk array. The network interface card 140 interfaces to a local area network (LAN) 142.

An expansion bus bridge 150 is also preferably coupled to the PCI bus 120. The expansion bus bridge 150 interfaces to an expansion bus 152. The expansion bus 152 may be any of a variety of types, including the industry standard architecture (ISA) bus, also referred to as the AT bus, the extended industry standard architecture (EISA) bus, or the microchannel architecture (MCA) bus. Various devices may be coupled to the expansion bus 152, including expansion bus memory 154 and a modem 156.

A bus arbiter 180 configured to control ownership of PCI bus 120 is illustrated as a portion of bus bridge 106 The arbitration scheme employed by PCI bus 120 employs a unique request signal REQ# and grant signal GNT# for each PCI master. When a particular master desires ownership of PCI bus 120, it asserts its associated request signal REQ#. For example, request signal REQ2 is asserted by video adapter 170 when it requires ownership of PCI bus 120, and request signal REQ5 is asserted by telephony adapter 174 when it requires ownership of PCI bus 120, and so on. Corresponding grant signals GNT 7:0! are routed back to the masters from bus arbiter 180 to indicate the current owner of PCI bus 120. Each request signal/grant signal pair is referred to as a bus request channel. It is noted that request signal REQ4 may be asserted by expansion bus bridge 150 if access of PCI bus 120 is required by an agent of expansion bus 152, such as modem 156.

In accordance with the invention, bus arbiter 180 is configured to dynamically vary a level of arbitration priority provided to a peripheral device based upon the length of time the peripheral device has been waiting to gain ownership of PCI bus 120. As a lapse of time increases from when the peripheral device requested the bus, the level of arbitration priority for that peripheral increases. The bus arbiter 180 is further configured such that certain peripheral devices may be assigned a fixed level of priority. Further details regarding bus arbiter 180 will be provided below in conjunction with the accompanying descriptions of FIGS. 2, 3A and 3B.

FIG. 2 is a diagram that depicts internal portions of bus arbiter 180. As shown, bus arbiter 180 includes an arbitration state machine 202 coupled to a request detection unit 204, a grant generator 208 and a request latency counters unit 210. A set of programmable registers 212 are further coupled between request latency counters unit 210 and a decoder 214.

Request latency counters unit 210 generates signals indicative of a lapse of time from which certain masters of PCI bus 120 requested the bus. FIGS. 3A and 3B illustrate one embodiment wherein a plurality of counters 210-0 through 210-7 are incorporated within request latency counters unit 210. Each counter unit 210-0 through 210-7 corresponds to a different master of PCI bus 120, and is associated with a separate bus request channel (i.e., counter unit 210-0 corresponds to request signal REQ0, etc.). Additional counter units may further be incorporated within request latency counters unit 210 for additional peripheral devices, as desired. Similarly, certain peripheral devices may not be associated with a counter unit, and thus fewer counter units may be employed.

Programmable registers 212 are provided to set and store the initial count value of each of the counters of request latency counters unit 210. The decoder 214 is provided to allow software programming of programmable registers 212. A separate storage region (shown in phantom in FIG. 2) is provided to set the initial count value of each counter unit. Programmable registers 212 may be mapped within the memory space, the I/O space, or the configuration space of the computer system.

Arbitration control unit 202 is provided to process incoming bus request signals detected by request detection unit 204, to prioritize contending requests, and to cause grant generator 208 to assert a grant signal to the winning master. Arbitration control unit 202 is configured such that the level of priority given to each master is dependent upon a corresponding count value of the counter unit associated with the master. The closer a particular counter unit is to a value of 0, the higher the priority level given by arbitration control unit 202. Different initial count values may be set for different counter units. For example, FIG. 3A illustrates a situation wherein the initial value of counter 210-0 is binary "1000", the initial value of counter 210-1 is binary "1100", the initial value of counter 210-2 is binary "1111 " and the initial value of counter 210-7 is binary "0110". As stated previously, the initial values of each counter is set by corresponding registers within programmable registers 212. Thus, for the moment in time when the values of counters 210 are as illustrated in FIG. 3 (and disregarding any other counters), arbitration control unit 202 assigns a highest level of priority to the request signal REQ7, since counter 210-7 conveys the lowest binary value. The second highest level of arbitration priority given to request signal REQ0, which corresponds to counter 210-0. Similarly, the next level of arbitration of priority is given to request signal REQ1, which is associated with counter 210-1, and request signal REQ2 is given the lowest priority.

Each of the counters 210-0 through 210-7 is configured to count down beginning at a time when the corresponding master requests the bus by asserting its REQ signal. If a request by a particular master is granted, the counter corresponding to that master is reset to its initial value, and again begins counting down if the master reasserts its bus request signal.

Each of the counters of request latency counters unit 210 may be inhibited from counting by setting an associated "hold bit" (H) within the corresponding storage region of programmable registers 212. When the hold bit for a particular counter unit is set, it retains its initial value programmed within programmable registers unit 212. Thus, a fixed level of arbitration priority may be set for selected peripheral devices, as desired. For example, it may be desirable to inhibit the counter unit of certain non-real time peripheral devices to set a fixed level of arbitration priority to the peripheral.

FIG. 3B illustrates an exemplary situation during the operation of the system wherein counter 210-0 has counted down to binary "0111", counter 210-1 has counted down to binary value "0110", and counter 210-2 has counted down to binary value "1000". For this example, the "hold" bit (H) associated with counter 210-7 was set, and thus the initial binary value of "0110" is maintained. For the moment in time as depicted in 3B, request signal REQ1 is given the highest level of arbitration priority since its count value is lowest. The next level of arbitration priority is provided to request signal REQ7, and so on. Thus, it is noted that for this moment in time, if the master associated with request signal REQ1 requires the bus, it will be provided first priority over any other contending masters. When the master is granted the bus, counter 210-1 is reset to its initial value of binary "1100", and resumes its count down if the master again requests the bus.

In accordance with the system described above, the counter units associated with real time bus masters which require quick access to the bus may be programmed with an initial count value which is relatively low. The counter units associated with real time bus masters which can withstand longer latencies in obtaining the bus may be programmed with initial count values which are higher. The initial count values of the counter units can thus be correlated to the maximum latency requirements of real time bus masters. The counter units associated with non-real time bus masters may be inhibited from counting to provide a fixed level of arbitration priority. As a result, overall improved system performance may be attained.

It is noted that bus arbiter 180 may further be configured to determine bus arbitration priority based upon various parameters in addition to the latency requirements of each bus master. For example, FIG. 4 illustrates an embodiment wherein a set of registers are provided to specify the bandwidth requirements for each master, the latency requirements for each master, and to specify whether a particular master is a real time or a non-real time resource. More specifically, FIG. 4 illustrates a set of registers 401-1 through 401-n that are provided to store the number of bytes to be passed per bus grant for each bus master. A set of registers 402-1 through 402-n are provided to specify the latency requirements for each bus master. These registers are similar to those described above in the embodiment of FIG. 2. A third set of registers 403-1 through 403-n are provided to convey information indicative of whether the resource is a real time or a non-real time device. If the resource is a non-real time device, additional bits in these registers may be provided to specify how much the device impacts system performance.

The latency information within registers 402-1 through 402-n are employed to control request latency counters similar to those described above in conjunction with FIG. 2. For the embodiment of FIG. 4, arbitration control unit 202 additionally uses the information regarding the number of bytes to be passed per bus grant as stored within registers 401 to determine whether it is necessary or possible to break an existing grant of the bus given to another device.

It is noted that a peripheral's drivers could update the associated registers of FIG. 4 based on the current requirements of the device. For example, an audio driver could change its bandwidth requirement register 401 based upon the number of voices (or other audio) being produced, and an MPEG driver could change its requirements based upon the size and frame rate of the MPEG window.

Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. 

What is claimed is:
 1. A bus arbiter for controlling and prioritizing ownership of a bus comprising:a request detection unit for detecting a plurality of bus request signals corresponding to designated bus masters; an arbitration control unit coupled to said request detection unit, wherein said arbitration control unit is configured to process contending bus requests received by said request detection unit and to control a granting of mastership of said bus to one of said designated bus masters; and a first counter unit coupled to said arbitration control unit wherein said first counter unit is configured to generate a first latency signal indicative of a first lapse of time following a request to obtain ownership of said bus by a first peripheral device which is one of said designated bus masters; a second counter unit coupled to said arbitration control unit wherein said second counter unit is configured to generate a second latency signal indicative of a second lapse of time following a request to obtain ownership of said bus by a second peripheral device which is another of said designated bus master; a programmable storage unit configured to store a first initial count value for said first counter unit and a second initial count value for said second counter unit; wherein said arbitration control unit is further configured to increase a level of arbitration priority given to said first peripheral device in response to said first latency signal indicating an increase in said first lapse of time; and wherein said arbitration control unit is further configured to increase a level of arbitration priority given to said second peripheral device in response to said second latency signal indicating an increase in said second lapse of time.
 2. The bus arbiter as recited in claim 1 wherein said programmable storage unit includes a plurality of registers.
 3. The bus arbiter as recited in claim 2 wherein each of said plurality of said registers is configured to store a parameter that controls whether an associated counter unit is inhibited from counting.
 4. The bus arbiter as recited in claim 1 wherein said programmable storage unit is further configured to store a bit to inhibit said first counter unit from counting.
 5. The bus arbiter as recited in claim 1 further comprising a decoder coupled to said programmable storage unit, wherein said decoder is configured to decode an address signal to thereby allow a setting of said first initial count value stored within said programmable storage unit.
 6. The bus arbiter as recited in claim 1, wherein said arbitration control unit coupled to said request detection unit is configured to control a granting of mastership of said bus to one of said designated bus masters based upon an arbitration algorithm.
 7. A computer system comprising:a microprocessor; a bus bridge coupled to said microprocessor through a CPU local bus; an expansion bus coupled to said bus bridge, wherein said bus bridge is configured to accommodate data transfers between said CPU local bus and said expansion bus; a real time master coupled to said expansion bus, wherein said real time master is configured to perform real time processing functions; a non-real time master coupled to said expansion bus, wherein said non-real time master is configured to perform non-real time processing functions; and a bus arbiter for controlling and prioritizing ownership of said expansion bus including: a request detection unit for detecting a first request signal of said real time master and a second request signal of said non-real time master; an arbitration control unit coupled to said request detection unit, wherein said arbitration control unit is configured to process contending bus requests received by said request detection unit and to control a granting of mastership of said expansion bus to either said real time master or said non-real time master; and a first counter unit coupled to said arbitration control unit wherein said first counter unit is configured to generate a first latency signal indicative of a first lapse of time following a request to obtain ownership of said expansion bus by said real time master; wherein said arbitration control unit is further configured to increase a level of arbitration priority given to said real time master in response to said first latency signal indicating an increase in said first lapse of time.
 8. The computer system as recited in claim 7, wherein said real time master is configured to assert a first request signal to request ownership of said expansion bus and wherein said non-real time master is configured to assert a second request signal to request ownership of said expansion bus.
 9. The computer system as recited in claim 7 wherein said expansion bus is a PCI bus.
 10. The computer system as recited in claim 7 wherein said real time master is an audio adapter.
 11. The computer system as recited in claim 7 further comprising a second counter unit coupled to said arbitration control unit wherein said second counter unit is configured to generate a second latency signal indicative of a second lapse of time following a request to obtain ownership of said expansion bus by said non-real time master.
 12. The computer system as recited in claim 11 wherein said arbitration control unit is configured to increase a level of arbitration priority given to said non-real time master in response to said second latency signal indicating an increase in said second lapse of time.
 13. The computer system as recited in claim 12 further comprising a programmable storage unit configured to store a first initial count value for said first counter unit and a second initial count value for said second counter unit.
 14. The computer system as recited in claim 13 wherein said programmable storage unit includes a plurality of registers.
 15. The computer system as recited in claim 14 wherein each of said plurality of said registers is configured to store a parameter that controls whether an associated counter unit is inhibited from counting.
 16. The computer system as recited in claim 13 wherein said programmable storage unit is further configured to store a bit to inhibit said first counter unit from counting.
 17. The computer system as recited in claim 13 further comprising a decoder coupled to said programmable storage unit, wherein said decoder is configured to decode an address signal to thereby allow a setting of said first initial count value stored within said programmable storage unit.
 18. The computer system as recited in claim 7, wherein said arbitration control unit is configured to control a granting of mastership of said expansion bus to either said real time master or said non-real time master based upon an arbitration algorithm. 